Difference between Install DVD & Upgrade DVD

[montycat]

I have both of these DVDs that Apple sent me after a received a defective Upgrade DVD. What is the difference between them? Thanks MC

 

[appledyl]

I'm guessing (though i'm not completely sure) that the Install DVD can install OS X to a mac despite the current operating system installed. An Upgrade CD requires the version you were upgrading from to install. (e.g. Upgrade from Tiger to Leopard = Upgrade CD. Upgrade from Mac OS 9 to Leopard = Install DVD)

Please correct me if I'm wrong.